1. Field of the Invention
The present invention relates to an EL (Electroluminescence) display comprising electroluminescence devices.
2. Description of the Related Art
In an organic EL display comprising many pixels composed of organic electroluminescence devices (referred to as "organic EL devices" hereinafter), a voltage is applied to the organic EL devices to inject electrons and holes to organic luminescent layers from cathodes and anodes, respectively, and recombination of the electrons and holes occurs in the organic luminescent layers to emit light.
As the organic EL devices provided in such an organic EL display, for example, the single hetero organic EL device shown in FIG. 8 can be used. This organic EL device comprises an anode 2 made of a transparent conductive film of ITO (Indium tin oxide), or the like and provided on a transparent substrate 1 such as a glass substrate; an organic layer 5 comprising a hole transport layer 3 and a luminescent layer 4 provided on the anode 2; and a cathode 6 made of aluminum or the like; which are provided in this order on the substrate 1.
On the basis of the above construction, in the organic EL device, when positive and negative voltages are applied to the anode 2 and the cathode 6, respectively, the holes ejected from the anode 2 reach the luminescent layer 4 through the hole transport layer 3, and the electrons ejected from the cathode 6 reach the luminescent layer 4 to produce recombination of the electrons and holes in the luminescent layer 4. At this time, light at a predetermined wavelength is produced, and emitted outward from the transparent substrate side, as shown by arrows in FIG. 8.
Therefore, many organic EL devices are arranged in, for example, a matrix form, to form the above-described organic EL display.
FIG. 9 shows an example of conventional such organic EL displays. The organic EL display shown in FIG. 9 comprises stripe transparent electrodes 8 provided as anodes on a transparent substrate 7, stripe organic layers 9a, 9b, and 9c each comprising a hole transport layer and a luminescent layer and provided on the transparent electrodes 8 perpendicularly thereto, and stripe cathodes 10 respectively provided on the stripe organic layers 9a, 9b, and 9c, and having substantially the same dimensions as the stripe organic layers 9a, 9b, and 9c. Each of the stripe organic layers 9a, 9b, and 9c has luminescent properties corresponding to one of red (R), green (G), and blue (B) so that the organic EL display becomes a full-color or multi-color display.
The image display of this color organic EL display will be described below. In the color organic EL display, as shown in FIG. 10 a scanning circuit 11 is connected to the transparent electrodes 8, and a luminance signal circuit 12 is connected to the cathodes 10. Signal voltages are applied to the organic layers 9a to 9c at the intersections of the transparent electrodes 8 and the cathodes 10 by the scanning circuit 11 and the luminance signal circuit 12 to emit light from the organic layers 9a to 9c. Therefore, such control causes the organic EL display to function as an image reproducer.
In order to manufacture the organic EL display shown in FIG. 9, conventionally, a plurality of the stripe transparent electrodes 8 comprising a transparent conductive film are formed in parallel on the transparent substrate 7. A physical deposition method such as sputtering or the like is frequency used for depositing the transparent conductive film. The transparent conductive film is processed to form stripes by conventional known lithography and etching techniques.
After the transparent electrodes 8 are formed, a plurality of the organic layers 9a to 9c, and the cathodes 10 are formed. The organic layers 9a to 9c, and the cathodes 10 are formed by vacuum deposition using a mask having stripe apertures. In a color display such as the organic EL display shown in FIG. 9, for example, the organic layers 9a corresponding to red (R), and the cathodes 10 formed thereon are deposited by using the same mask, and then the mask is changed (or the mask is moved) so that the organic layers 9b and 9c corresponding to green (G) and blue (B), and the cathodes 10 formed thereon are formed by the same method.
However, such a conventional manufacturing method employs the mask deposition method for forming the patterns of the organic layers 9a to 9c, and the cathodes 10, and thus has the disadvantage described below.
Namely, the pattern formation by the mask deposition method has limits of alignment precision of the mask, and processing precision of the mask, and is difficult to form a fine pattern, thereby causing difficulties in producing a high-definition display.
As a method of manufacturing an organic EL display, which is capable of forming a fine pattern with high precision, and solving the above disadvantage, the technique disclosed in Japanese Unexamined Patent Publication No. 9-293589 is known.
This method comprises the lamination step of laminating in turn an organic film, a cathode material film, and a protecting film on a transparent substrate on which cathodes (transparent electrodes) made of a transparent conductive material are formed, the resist pattern forming step of forming a resist pattern in a desired shape on the protecting film, and the patterning step of processing the laminated film comprising the organic film, the cathode material film and the protecting film by a dry etching method using the resist pattern as a mask to form organic layers, cathodes and protecting layers in a desired shape. The steps of coating a resist on the protecting film and processing the laminated film by dry etching are repeated at least once to finely pattern the organic layers and cathodes.
However, such a method also has the disadvantages below.
In dry-etching the protecting film, the cathode material film, the organic film using the resist pattern as the mask, the etching residue of the cathode material film having conductivity (electric conductivity) adheres to the etching-side walls of the organic layers made of the organic film, thereby causing the probability of causing short circuits between the obtained cathodes and anodes. The occurrence of the short circuits between the cathodes and the anodes makes impossible the selection of a pixel in the display, and thus makes it impossible for the resultant organic EL display to function as a display.
In the method of manufacturing the organic EL display, the step of depositing the organic film, the cathode material film, the protecting film, and the resist film, and the step of processing the laminated film comprising the organic film, the cathode material film, and the protecting film by dry etching are repeated to pattern the organic layers. Therefore, the organic layers are formed below the stripe cathodes along the length direction thereof. For example, in manufacturing a simple matrix organic EL display, thus, the electrodes on the scanning side are necessarily anodes.
However, in the simple matrix system, a current of 100 to 1000 times as large as the current of the signal side electrodes flows through the scanning-side electrodes, and the voltage drop in the scanning electrodes is significantly increased because the scanning-side electrodes serving as anodes are made of a material such as ITO or the like, which has resistance of about 100 times as high as metals. Such an increase in voltage drop in the scanning electrodes causes variations in luminance of the display, and an increase in power consumption.
In the organic EL display obtained by the above-described method, the organic layers are formed over the lower sides of the cathodes along the length direction thereof, thereby causing the disadvantage below.
Since the organic layers have photoconductivity (optical waveguide), a part of light produced in the organic layers 9 is guided transversely to the transparent substrate 7 through the organic layers 9, as shown by arrows in FIG. 11. As a result, part of the light is attenuated during guide, the residue being released from the peripheral pixels and lost.
Namely, it is preferable that the light emitted in the organic layers 9 are entirely passed through the transparent electrodes 8 and the transparent substrate 7 and emitted to outside of the organic EL display to be used as display light. However, in the organic EL display obtained by the conventional manufacturing method, part of the light emitted in the organic layers 9 is not used as display light, decreasing the efficiency of utilization of light, and decreasing luminance.
Also, part of the light transversely guided to the transparent substrate 7 through the organic layers 9 is released from the organic layers 9, which constitute the peripheral pixels, and emitted to the outside of the transparent substrate 7, and thus cross talk occurs due to interference in the originally produced light, causing the probability of deterioration in color reproducibility.